Cleaning of polluted gas with a view to removing particulate or gaseous substances is an important and common process in today's industrialized society. A vast variety of techniques have been developed, and today there are often several methods to choose between when a gas cleaning plant is to be designed, even when very specific pollutants are to be removed.
Particulate pollutants are often removed by means of dynamic separators, such as cyclones, electrostatic precipitators or barrier filters, bag filters or cassette filters.
Gaseous pollutants are generally removed by the roundabout technique of using some additive for convening the gases into particulate substances, either by binding them to the surface of particles supplied, dry or wet, or by reacting them with substances supplied, also in gaseous or liquid form, so as to obtain a particulate product. The reaction product is thereafter separated in a particle separator.
Cooling gas with a view to adapting its temperature or recovering heat therefrom is also nowadays an important and common process. Heat transfer generally takes place either by means of heat exchangers of recuperative or regenerative type or by direct contact between the hot and the cold medium. Since this invention concerns heat transfer by direct contact between a gas and a liquid, other techniques will not be discussed.
One method which is advantageous in many respects consists in conducting a gas through a rain of finely divided liquid or past surfaces overflowed by a liquid. These methods make it possible to cool a hot gas as well as to capture particles in the liquid and to dissolve gaseous components of a polluted gas in the liquid. The liquid may then also contain substances convening the dissolved gaseous components into solid form in order to make it easier to separate them from the liquid.
The liquid is normally recycled in the washing device, but a portion thereof is removed, generally continuously, in order to use its heat in other applications or to be subsequently treated for separating pollutants, either in gaseous form or in solid form, optionally for recovering the substances, and the thus cooled or in other way treated liquid can be recycled to the gas washing plant to be used again.
These gas washing plants can roughly be divided into open towers where the gas only encounters a finely divided liquid, and packed scrubbers or packed columns where the gas flows through a tower filled with e.g. saddle-shaped or coil-shaped, small parts, on to which liquid is sprayed so as to produce a liquid film which flows downwards over essentially the entire total surface.
Since packed scrubbers do not fall within the field of application of the present invention, they will not be discussed here.
Examples of open towers, e.g. for separating sulphur dioxide and cooling a gas in order to recover heat, are given in e.g. U.S. Pat. No. 3,532,595, where both vertical towers and scrubbers with horizontal gas flow are disclosed and liquid is supplied at several levels or positions. U.S. Pat. No. 4,164,399 describes a tower of less complex design, where liquid is supplied only at one level but is distributed after being captured at several levels. U.S. Pat. No. 2,523,441 shows a combination of an open tower with a packed section.
The above-mentioned techniques substantially require that the liquid used in the gas washer, during the major part of its movement in the tower, falls or flows downwards by gravity. It is however also known to design scrubbers which generate more or less horizontal liquid curtains through which the gas is flowed. One example of this is found in the highly complex design disclosed in SE-103,474, where the descending movement of the gas is assumed largely to take place along the vertical walls. Two other examples are given in U.S. Pat. No. 2,589,956 and U.S. Pat. No. 3,691,731.
An intermediate design is disclosed in U.S. Pat. No. 4,583,999, where the washing liquid is supplied horizontally but, probably after some deceleration, descends as a rain of finely divided droplets.
In a gas washing tower of the type closest the invention in the known "State of the art", e.g. DE-A1 33 41 318 or U.S. Pat. No. 3,532,595, liquid is generally supplied at 4-6 levels. Each level has several nozzles distributing small droplets within an area generally in the form of a conical shell, hollow-cone type, or within a complete cone, full-cone type. The vertex angle of this cone is 90.degree.-120.degree..
Each level is provided with nozzles arranged with a spacing of 0.5-1 m, in a regular lattice. The distance between the levels is 1-2 m. At least some levels are located far above the bottom of the tower. The purpose of this is that these levels should produce droplets which in the form of a well-distributed rain descend through the tower throughout a considerable part of its height.
The efficiency of the gas washer is largely dependent on the relative movement between the droplets and the gas. It is therefore generally preferred that the gas flows upwards in a direction contrary to the descending droplets, i.e. countercurrently, but for various reasons there also exist gas washers in which the gas descends in the same direction as the descending droplets, i.e. concurrently.
If it is desirable to increase the gas washing efficiency when using this method, it is necessary either to increase the height of the tower or to increase the flow of washing liquid. Whichever option is chosen, the consequence is increased pump work for a given volume of gas flow.